Demographic Responses of Small Mammals to Disturbance Induced by Forest Management
Disturbance induced by forest management aimed at oak (Quercus ) regeneration alters environmental conditions that have consequences for forest organisms. Populations of many small mammals including eastern chipmunks (Tamias striatus), white-footed mice ( Peromyscus leucopus), and short-tailed shrews (Blarina brevicauda ) respond quickly to changes in microhabitat following timber harvesting. However, few studies have assessed density responses, especially at time lags of several years following harvest. I collected small mammal mark-release-recapture data 6-8 years post-harvest in the Hardwood Ecosystem Experiment in southern Indiana, modelled density (for chipmunks and mice) and relative abundance (shrews) as a function of habitat type (matrix, harvest opening, opening edge, and shelterwood) and examined the relationship between density and distance to harvest boundary with spatially explicit capture-recapture models. Chipmunks responded positively to all types of harvest openings. Matrix habitat supported 58 and 71% lower densities of chipmunks relative to harvest openings and opening edges, respectively. Chipmunks increased in density following the second stage of a shelterwood harvest. White-footed mouse density decreased following a poor mast year, and both mouse and chipmunk densities were elevated in the summer following a relatively good mast year. I observed distance-dependent density relationships for both white-footed mice and chipmunks in which both species tended to exhibit higher densities near harvest boundaries relative to forest matrix. Structural complexity created at the edge of harvest openings offers benefits to species associated with edge habitat, especially 6-8 years after harvest. Short-tailed shrews responded negatively to all harvest treatments, especially clearcuts (37% of control relative abundance), but shrew use of openings appears to have increased 6-8 years post-harvest. Prescribed fire increasingly is used to promote regeneration of economically and ecologically valuable oaks (Quercus) in North American forests. However, fire as a disturbance may have implications for other forest organisms, particularly those that inhabit the forest floor and rely upon habitat variables such as leaf litter, acorns, and understory vegetation. Small mammals can be useful indicators of the effects of management activities such as prescribed fire and are important to consider due to their roles in forest trophic interactions. I estimated the effects of prescribed burns in oak-hickory forests of southern Indiana on density and movements of white-footed mice and chipmunks with spatially explicit capture-recapture models and compared relative abundance of shrews using a before-after-control-impact design with sampling 3-18 months (spring) and 3-11 months (fall) following seasonal prescribed burns. Small mammal responses varied according to species, burn season, and time since fire. Relative to controls, predicted eastern chipmunk density was 39% higher in the period immediately following spring burns and 51% following fall burns, but the positive effect of fire decreased with time. Movements of eastern chipmunks increased in magnitude the growing season immediately following both spring and fall burns but were reduced to levels similar to controls by the next fall. White-footed mice did not exhibit a detectable change in movements or population density to spring burns, but relative abundance increased by 78% relative to controls following a fall burn. Relative abundance of short-tailed shrews was unchanged relative to controls following a spring burn but was negatively impacted by fall burns. Small mammal populations exhibited varied responses to spring and fall burns, but effects of burning generally diminished with increased time since fire. These results suggest that timing should be carefully considered when implementing fire as a management tool, both in terms of burn season and length of time available for habitat recovery. Information from estimates of vital rates is critical to a robust assessment of habitat quality for wildlife populations. I assessed small mammal vital rates in response to shelterwood, clearcut, patch cut, and spring and fall prescribed burns in southern Indiana as part of the Hardwood Ecosystem Experiment. I collected small mammal mark-release-recapture data and revisited sites seasonally to estimate survival and temporary emigration of eastern chipmunks and white-footed mice. I compared candidate models using an information-theoretic approach to determine effects of treatment and time. The top-ranked model for white-footed mice in timber harvest units included effects of treatment and time, with monthly survival rates reduced by 13% in shelterwoods and 17% in patch cuts relative to controls. Timber harvests did not have a significant effect on chipmunk survival 6-8 years post-treatment. In areas subjected to prescribed fire, time did not have an effect on survival of chipmunks or mice, but temporary emigration varied by time and treatment.
Swihart, Purdue University.
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